In mobile communication systems or packet communication systems using IP, the limitation of transmission bandwidth and the digital signal processing speed of DSP (Digital Signal Processor) is gradually becoming less important. If a transmission rate becomes a higher bit rate, bandwidth for transmitting a plurality of channels can be ensured, so that communication employing stereo schemes (stereo communication) is expected to be common even in speech communication where mono schemes are major streams.
A current mobile phone has already integrated a multimedia player and FM radio functionality which provide stereo capability. Therefore, it will be a natural extension to add stereo capability to the fourth generation mobile phones and IP telephones to record and playback not only stereo audio signals but also stereo speech signals.
There are many methods to encode stereo signals. Non-Patent Document 1 discloses a representative method called “MPEG-2 AAC” (Moving Picture Experts Group-2 Advanced Audio Coding). MPEG-2 AAC can encode signals in mono, stereo and multiple channels. MPEG-2 AAC performs MDCT (Modified Discrete Cosine Transform) processing to convert time domain signals into frequency domain signals. Further, MPEG-2 AAC exploits the human auditory system to generate good sound quality such that the coding artifacts are masked and kept below a human hearing threshold.    Non-Patent Document 1: ISO/IEC 13818-7:1997-MPEG-2 Advanced Audio Coding (AAC)